WO2024113431A1 - Method for improving strength, toughness and uniformity of ultra-large-section non-quenched and tempered steel by cooperative regulation and control of multi-stage precipitation and controlled rolling and controlled cooling - Google Patents

Abstract

Disclosed is a method for improving strength, toughness and uniformity of ultra-large-section non-quenched and tempered steel by cooperative regulation and control of multi-stage precipitation and controlled rolling and controlled cooling. The method comprises the following steps: S1, performing continuous heating before rolling of a continuous casting billet; S2, performing water descaling after the continuous casting billet is taken out of a furnace, performing water cooling or waiting until a surface temperature is lower than 1100°C and then starting rolling; and S3, cogging rolling: rolling two perpendicular surfaces of the continuous casting billet in the first two processes and pressing down 5-10 mm at each process, alternately rolling the two perpendicular surfaces of the continuous casting billet in the following 4-6 consecutive processes, wherein the amount of deformation at each process is larger than or equal to 20%, and then performing controlled finish rolling and controlled cooling to obtain a finished rolled product. According to the present invention, by cooperatively regulating and controlling the content and proportion of alloy and a controlled rolling and controlled cooling process, precipitated phases are precipitated in multiple stages, and match structure evolution during rough rolling, finish rolling, and post-rolling cooling, thereby refining high-temperature and room-temperature structures of a round steel core. The strength, toughness and section uniformity of the ultra-large-section non-quenched and tempered steel for direct cutting and with a diameter of 160-300 mm are improved.

Description

A method for synergistically controlling the strength, toughness and uniformity of ultra-large cross-section non-quenched and tempered steel by multi-stage precipitation and controlled rolling and controlled cooling Technical Field

The invention relates to a method for cooperatively regulating the strength, toughness and uniformity of ultra-large cross-section non-quenched and tempered steel by multi-stage precipitation and controlled rolling and controlled cooling, belonging to the technical field of metal materials.

Background technique

Compared with quenched and tempered steel, non-quenched and tempered steel has the advantages of low cost, green environmental protection, and friendly operation. It is widely used in the fields of automobiles, engineering machinery, etc. Compared with non-quenched and tempered steel for hot forging, non-quenched and tempered steel for direct cutting can achieve the required mechanical properties while rolling the round steel into shape, further shortening the production process. Usually, it can be used after only undergoing processes such as smelting and solidification, continuous rolling into round steel, and downstream cutting processing, and the cost and environmental advantages are further improved. At present, the production of non-quenched and tempered steel for direct cutting with higher strength and toughness matching and larger cross-sectional area is an important development direction with broad application prospects. However, as the cross-sectional area increases, the difficulty of improving the strength and toughness of non-quenched and tempered steel and ensuring the uniformity of the cross section increases significantly.

The large temperature difference between the center and the surface of the ultra-large cross-section non-quenched and tempered steel during rolling, poor deformation penetration, and small area reduction before and after rolling can easily lead to coarse microstructure in the center of the rolled round steel, poor strength and toughness; the difference between the center and the surface microstructure is obvious, and the cross-sectional uniformity is poor. This makes the surface machining of the round steel difficult and the core performance does not meet the standard. The main reasons for the large difference between the center and the surface of the ultra-large cross-section round steel are: (1) poor deformation penetration during high-temperature rolling, small deformation in the center, coarse recrystallized grains; large deformation on the surface, small recrystallized grains. (2) During medium and low temperature rolling, the center temperature is high and in the recrystallization zone, the austenite grains will still grow, and the new phase will only nucleate at the austenite grain boundaries during cooling, resulting in a coarse microstructure; while the surface temperature is low and in the non-recrystallization zone, the defects of the deformed austenite grains increase significantly, and the new phase can nucleate at the austenite grain boundaries and internal defects during cooling, resulting in a fine microstructure. (3) During cooling after rolling, the temperature drop in the center is small, the transformation temperature is high, and the pearlite lamella spacing is large; the surface temperature drop is large, the transformation temperature is low, and the pearlite lamella spacing is small. The above three factors all lead to coarse core structure and fine surface structure of super-large cross-section non-quenched and tempered steel; the gap between the core and the surface increases significantly after the superposition of the three factors.

It is a common method to use microalloying elements such as Ti, Nb, V, N, and Al to control the microstructure and product properties of non-quenched and tempered steel. However, the effect is mainly focused on the controlled forging and controlled cooling process of parts after blanking of non-quenched and tempered round steel for hot forging. There is little research on its role in rolling round steel for direct cutting and its influence on the microstructure and properties of round steel. There are significant differences between the rolling process and the forging process.

Chinese Patent Publication No. CN113621882A controls the mechanical properties and surface quality of round steel by controlling the content and proportion of microalloying elements, but does not involve precise control of the rolling process. Chinese Patent Publication Nos. CN104043660A and CN114472519A improve the cross-sectional uniformity of non-quenched and tempered steel by controlling the water penetration process after round steel finish rolling, but do not consider the specific role of different microalloying elements in the steel rolling process. Chinese Patent Publication Nos. CN113122776A, CN113122776A, and CN113134510B provide a composition range and rolling process for non-quenched and tempered steel for direct cutting of medium and large cross-sections, but do not consider the role of coordinated control of precipitation phase and rolling process. Moreover, none of the above patents consider the organizational evolution characteristics of ultra-large cross-section non-quenched and tempered round steel (diameter above 160 mm) during the rolling process.

Therefore, for non-quenched and tempered steels for direct cutting of ultra-large cross-section ferrite-pearlite, a method is needed to optimize strength, toughness and cross-sectional uniformity by synergistically regulating the precipitation of microalloying elements and controlled rolling and controlled cooling processes.

Summary of the invention

The technical problem to be solved by the present invention is to obtain higher strength, toughness and cross-section uniformity for ultra-large cross-section ferrite-pearlite direct cutting non-quenched and tempered steel with a diameter of 160-300 mm by regulating the synergistic effect of the multi-stage precipitation process of micro-alloy elements and the controlled rolling and controlled cooling process.

In order to solve the above technical problems, the technical solution adopted by the present invention is:

A method for collaboratively controlling the strength, toughness and uniformity of ultra-large cross-section non-quenched and tempered steel by multi-stage precipitation and controlled rolling and controlled cooling, specifically comprising the following steps:

S1: Continuous heating before rolling of continuous casting billet, wherein the temperature of the second heating stage is 1230℃～1250℃, the soaking stage is 1210℃～1230℃, and the total holding time of the second heating stage and the soaking stage is not less than 5h, to ensure that all micro-alloy elements except part of TiN are dissolved into austenite, and the undissolved TiN can inhibit the growth of austenite grains;

S2: After the continuous casting billet is taken out of the furnace, it is descaled by water, and then rolled after being cooled by water or heated to a surface temperature below 1100°C, so as to ensure that the surface hardness of the continuous casting billet is significantly higher than the high-temperature metal in the core;

S3: Use reversible rough rolling mill to roll the billet. The first two passes are to roll the two vertical surfaces of the continuous casting billet, and each pass is pressed down by 5-10mm. The following 4 to 6 consecutive passes are alternately rolled on the two vertical surfaces of the continuous casting billet. The deformation amount of each pass is ≥20%. The subsequent passes do not require deformation. It can be rolled into an intermediate billet. The final rough rolling temperature is greater than 1050℃;

S4: The intermediate billet is water-cooled or heated to 900-950°C on the surface, during which all TiN is precipitated, and NbN and AlN are partially precipitated;

S5: The intermediate billet is rolled into round steel by finishing continuous rolling, with the final rolling temperature at 780℃～820℃, during which the nitrides of Ti, Nb and Al are all precipitated;

S6: After the round steel is finished rolled, it is quickly cooled by water. After 6-8 sets of strong water cooling, the surface temperature is cooled to 550-600℃. During this period, air cooling is interspersed. The temperature return time is 5-10s each time. The temperature return will be performed between every two sets of water tanks. The surface temperature drop of the round steel after each set of strong water cooling is not more than 100℃. After that, it is sprayed with water mist to maintain the surface temperature of the round steel at 550-600℃. During this period, V is mainly precipitated in the form of VC.

S7: After the round steel is cooled to 300-350℃ on the surface by a walking cooling bed with straightening function, it is put into the pit for insulation for at least 48 hours to become the finished product.

The method for cooperatively regulating the strength, toughness and uniformity of ultra-large cross-section non-quenched and tempered steel by multi-stage precipitation and controlled rolling and controlled cooling, wherein the chemical composition of the round steel is as follows, in percentage by mass, C: 0.35% to 0.45%, Si: 0.30% to 0.90%, Mn: 1.10% to 1.60%, V: 0.10% to 0.20%, Nb: 0.03% to 0.04%, Ti: 0.015% to 0.030%, Al: 0.10% to 0.30%, N: 0.008% to 0.020%, S: 0.01% to 0.04%, P≤0.02%, and the remainder is Fe and unavoidable impurities.

Preferably, the mass percentage of Ti is 0.020% to 0.028%, the mass percentage ratio of Ti/N is less than 3.4, and the mass percentage ratio of (Ti+Nb+Al)/N is greater than 12.5.

The method for cooperatively regulating the strength, toughness and uniformity of ultra-large cross-section non-quenched and tempered steel by multi-stage precipitation and controlled rolling and controlled cooling is aimed at square billets or rectangular billets with a side length of 400-1000 mm for continuous casting billets, square billets with a side length of 230-450 mm for intermediate billets, and round steels with a diameter of 160-300 mm after rolling, longitudinal tensile strength of 900-1050 MPa at 1/4 radius and core, yield strength of 650-800 MPa, elongation after fracture of 15-19%, and impact energy KU ₂ of 40-50 J.

The present invention has the following new features: (1) By synergistically regulating the types and contents of alloying elements in non-quenched and tempered steel and the controlled rolling and controlled cooling process, different types of precipitated phases are graded and precipitated in accordance with the organizational evolution during hot rolling and subsequent cooling, thereby improving the strength and toughness while also improving the cross-sectional uniformity of the round steel. (2) The rough rolling process adopts high-temperature heating, low-temperature rolling, and continuous single-pass large deformation technology to fully realize deformation penetration and improve the cross-sectional uniformity of the rough rolling process. (3) The finishing rolling adopts a pre-rolling warm-up and low-temperature rolling process so that the entire cross-section is rolled in the non-recrystallized zone. (4) After finishing rolling, a strong water cooling + air cooling alternating cooling + water mist temperature control cooling mode is adopted to improve the uniformity of the room temperature organization and performance of the entire cross-section.

The role and proportion of the elements in the present invention are as follows:

C: basic element in non-quenched and tempered steel. In addition to forming a sufficient proportion of pearlite, part of it enters the carbonitride of microalloying elements to play the role of fine grain strengthening and precipitation strengthening. If the carbon content is too low, the strength of the steel part is insufficient. If it is too high, the elongation and impact energy of the steel part are insufficient. Therefore, the C content of the present invention is 0.35% to 0.45%.

Si: basic element for strengthening ferrite. As the content of Si increases, the strength of steel parts increases, but the plasticity decreases. The Si content of the present invention is 0.30% to 0.90%.

Mn: strengthens the matrix, increases the pearlite ratio, refines the pearlite interlamellar spacing, and improves toughness; forms MnS to improve cutting performance. The Mn element content of the present invention is 1.10% to 1.60%.

Ti: Combined with N, it precipitates TiN to refine austenite grains. When kept at 1250°C for a long time, a large amount of TiN remains undissolved, preventing the austenite grains from growing, thereby refining the room temperature structure and improving toughness. The general addition amount in non-quenched and tempered steel is 0.01% to 0.02%. The heating temperature before rolling of ultra-large cross-section non-quenched and tempered steel is high and the holding time is long. More Ti is required to ensure that the grains are not coarse during high-temperature equalization. Therefore, the Ti element content of the present invention is 0.015% to 0.030%. Preferably, the mass percentage of Ti is 0.020% to 0.028%.

Nb: Increases the recrystallization temperature of austenite and refines the room temperature structure. Non-quenched and tempered steel usually adds about 0.02-0.03% Nb. Only when there are enough solid-dissolved Nb atoms can the recrystallization temperature of austenite be increased to about 1000°C. In order to ensure that the entire cross-section of the intermediate billet is in the unrecrystallized zone of austenite during finish rolling, it is necessary to increase the content of solid-dissolved Nb elements in the steel. Therefore, the Nb element content of the present invention is 0.030% to 0.040%.

Al: In addition to deoxidation, AlN is formed to produce fine grain strengthening and precipitation strengthening, and usually begins to precipitate in large quantities at temperatures below 1100° C. In order to improve the effect of grain refinement during rough rolling, the Al content of the present invention is 0.10% to 0.30%.

V: Main element for precipitation strengthening. The precipitation temperature of VN is about 900℃, while that of VC is 700-800℃. VC with lower precipitation temperature is more fine and dispersed. As the strength level increases, the amount of V added in ferrite-pearlite non-quenched and tempered steel gradually increases, generally up to about 0.1%. The cooling rate of super-large cross-section non-quenched and tempered steel is slow after rolling, and the V content needs to be appropriately increased to form sufficient carbonitrides to ensure strength. Therefore, the V content of the present invention is 0.10% to 0.20%.

N: forms carbonitrides of microalloying elements to refine austenite grains and precipitate to produce second phase strengthening. The atomic weight ratios of Ti, Nb, Al and N are 3.4, 6.6 and 1.9 respectively. The solid solution temperature of nitrides is significantly higher than that of carbides of the same microalloying elements. To ensure the effect of grain refinement during heating and rough rolling, all Ti in the steel exists in the form of nitrides, and the Ti/N mass percentage ratio should be less than 3.4. At the same time, in order to obtain more fine and dispersed VC, it is necessary to ensure that N elements are fixed by Ti, Nb and Al elements as much as possible, and the (Ti+Nb+Al)/N mass percentage ratio is greater than 12.5. Therefore, the N content of the present invention is 0.008% to 0.020%.

S: Combined with Mn to form MnS, it improves the cutting performance of non-quenched and tempered steel. The general addition amount in non-quenched and tempered steel is 0.03-0.40%. Too high S will damage the transverse mechanical properties, so the S content needs to be appropriately reduced. Therefore, the S element content of the present invention is 0.01%-0.04%.

P: impurity element in steel, promotes cold brittleness, so the total content in ultra-large cross-section non-quenched and tempered steel needs to be strictly controlled. Therefore, the P element content of the present invention is not more than 0.02%.

The beneficial effects of the present invention are mainly reflected in:

(1) For non-quenched and tempered steel for direct cutting with an ultra-large cross-section and a diameter of 160-300 mm after rolling, the present invention coordinates and regulates the multi-level precipitation of microalloying elements and the controlled rolling and controlled cooling process, giving full play to the role of TiN, NbN and AlN in refining grains during heating and rough rolling of the ingot, the role of solid solution Nb in increasing the temperature of the recrystallization zone during finishing rolling, and the precipitation strengthening role of VC during controlled cooling after rolling, fully refining the core structure and VC size of the round steel, and significantly improving the uniformity of the core and surface structure and performance while improving the toughness of the core.

(2) The present invention realizes dynamic recrystallization rolling of austenite in the whole cross section during the whole rough rolling process by synergistically regulating the multi-level precipitation of microalloying elements and the controlled rolling and controlled cooling process, so as to distribute more deformation to the core, improve the degree of core refinement and core-surface uniformity. The non-dynamic recrystallization zone rolling of austenite in the whole cross section during the whole finishing rolling process distributes more deformation to the core, improves the core defect density and core-surface uniformity. Post-rolling cooling reduces the core temperature as quickly as possible, so that the core and the surface are transformed under the same original structure and temperature conditions, thereby improving the strength and toughness while improving the uniformity of room temperature structure and performance.

The present invention discloses a method for cooperatively regulating the strength, toughness and uniformity of ultra-large cross-section non-quenched and tempered steel by multi-stage precipitation and controlled rolling and controlled cooling. By cooperatively regulating the content and proportion of Ti, Nb, Al, V and N and the controlled rolling and controlled cooling process, multi-stage precipitation of the precipitated phase is achieved, and the microstructure evolution during rough rolling, fine rolling and post-rolling cooling is matched to refine the high-temperature and room-temperature microstructures of the core of the round steel. The strength, toughness and cross-section uniformity of ultra-large cross-section non-quenched and tempered steel for direct cutting with a diameter of 160-300 mm are improved.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is a microstructure diagram of the rolled material of Comparative Example 1, wherein (a) is the microstructure at a radius of 1/4 from the surface, and (b) is the microstructure at the core;

FIG2 is a microstructure diagram of the rolled material of Example 1, wherein (a) is the microstructure at a radius of 1/4 from the surface, and (b) is the microstructure at the core;

FIG3 is a microstructure diagram of the rolled material of Example 2, wherein (a) is the microstructure at a radius of 1/4 from the surface, and (b) is the microstructure at the core;

FIG4 is a diagram of undissolved TiN in a continuous casting billet after heating, wherein (a) is a morphology diagram and (b) is an elemental analysis diagram;

FIG5 is a diagram of TiN and NbN precipitated during the rough rolling process, wherein (a) is a morphology diagram and (b) is an element analysis diagram;

FIG6 is a diagram of TiN, NbN and AlN precipitated during water cooling or warming of the intermediate billet, wherein (a) is a morphology diagram of TiN and NbN, (b) is an elemental analysis diagram of TiN and NbN, (c) is a morphology diagram of AlN, and (d) is an elemental analysis diagram of AlN;

FIG7 is a diagram of NbN and AlN precipitated during finishing continuous rolling, wherein (a) is a morphology diagram, (b) is an elemental analysis diagram of NbN, and (c) is an elemental analysis diagram of AlN;

Figure 8 is a diagram of VC precipitated during the cooling stage after rolling, where (a) is a morphology diagram and (b) is an elemental analysis diagram;

Figure 9 shows the morphology of pearlite lamellar spacing in round steel products.

Detailed ways

The present invention will be further described in detail below in conjunction with the accompanying drawings and specific embodiments. The following embodiments are only used to illustrate the present invention and are not intended to limit the scope of the present invention.

Example 1

A method for cooperatively controlling the strength, toughness and uniformity of ultra-large cross-section non-quenched and tempered steel by multi-stage precipitation and controlled rolling and controlled cooling, wherein the continuous casting billet used has the following chemical composition by mass percentage: 0.38% C, 0.60% Si, 1.39% Mn, 0.01% P, 0.03% S, 0.020% Ti, 0.033% Nb, 0.12% Al, 0.15% V, 0.012% N, and the remainder is Fe and unavoidable impurities, wherein Ti/N=1.67, (Ti+Nb+Al)/N=14.42. The cross-sectional size of the continuous casting billet is 430mm×480mm.

A method for multi-stage precipitation and controlled rolling and controlled cooling to coordinately control the strength, toughness and uniformity of ultra-large cross-section non-quenched and tempered steel. The continuous casting billet is heated by a walking beam heating furnace, wherein the second heating stage is 1250℃, the soaking stage is 1230℃, and the second heating stage + soaking stage are kept warm for 5.5h. After water descaling, water cooling is carried out until the surface temperature reaches 1080℃, and rolling is carried out by a reversible rough rolling mill. The first two rolling passes press down the two vertical surfaces of the continuous casting billet by 10mm respectively, and the latter two surfaces are alternately pressed down by 100mm, for a total of 5 passes. The subsequent passes are rolled into a square billet with a side length of 240mm, and the final rolling temperature is 1080℃. The intermediate billet is water-cooled to a surface temperature of 930℃ and then rolled into a round steel with a diameter of 180mm in the finishing continuous rolling unit. The final rolling temperature is 800℃. After 6 groups of interval strong water penetration, water mist is sprayed to a surface temperature of 570℃, and then air-cooled on the cooling bed, and then kept in the pit at 330℃ for 48h to become a finished product.

The microstructure level and mechanical properties of the round steel core and 1/4 radius position using this embodiment are shown in Table 1.

The principle of coordinated regulation of multi-stage precipitation of microalloying elements and controlled rolling and controlled cooling in this embodiment is as follows:

(1) In order to ensure that the core structure of the ultra-large cross-section continuous casting billet is fine during rough rolling and reduce the unevenness between the core and the surface, the following measures are taken:

① Increase the heating and soaking temperature of the continuous casting billet to reduce the deformation resistance of the core of the billet; at the same time, add enough TiN to prevent the growth of austenite grains during heating.

② After water descaling, the continuous casting billet is water-cooled or heated to 1080℃ on the surface, and the surface hardness increases, but the temperature of the core of the continuous casting billet is almost not reduced, and the hardness is low. The surface deformation is small during rough rolling, so that more deformation occurs in the core, and the austenite grains in the core are fully refined. At the same time, the gradually precipitated TiN and a small amount of NbN can also prevent the growth of austenite grains during hot rolling.

③ The first two passes of rough rolling use small deformation to remove surface oxide scale and other defects, and the subsequent large deformation ensures that the deformation fully penetrates into the core and refines the austenite grains in the core.

④ The final rolling temperature of rough rolling is greater than 1050℃ to ensure that both the core and the surface are rolled in the austenite recrystallization zone.

The main purpose of the measures in the continuous casting billet heating and rough rolling stages is to refine the austenite grains and improve the toughness and cross-sectional uniformity of the round steel. As shown in Figure 4, 3-5μm TiN is retained in the continuous casting billet heating stage, which plays a role in refining the original structure and improving toughness. As shown in Figure 5, 1-3μm TiN and NbN are precipitated during the rough rolling process, which plays a role in refining the recrystallized grains and improving toughness, and can also play a certain precipitation strengthening role.

(2) In order to ensure that the core structure of the intermediate billet is fine during finish rolling and reduce the unevenness between the core and the surface, the following measures are taken:

① The intermediate billet is water-cooled or heated to 930℃ on the surface. During this period, all TiN is precipitated, and NbN and AlN are partially precipitated, which increases the defect density during finishing rolling and refines the room temperature structure. At the same time, the Nb element in the core is retained in solid solution, which increases the austenite recrystallization temperature, so that the core and surface are both in the austenite non-recrystallization zone during finishing rolling, reducing the difference in the evolution of the core and surface structures. In addition, the temperature makes the intermediate billet surface hard and the core soft, which is conducive to deformation penetration into the core and refinement of the core structure.

②Use finishing continuous rolling with a final rolling temperature of 800℃, during which the nitrides of Ti, Nb and Al are all precipitated, further strengthening the interaction between the precipitated phase and deformation defects, increasing the defect density, refining the room temperature structure, and improving the strength and toughness of the round steel.

The main purpose of the measures in the intermediate billet warming and finishing rolling stages is to refine the room temperature structure and improve the toughness and cross-sectional uniformity of round steel. As shown in Figure 6, 500nm-1μm TiN, NbN and AlN are precipitated in the intermediate billet warming stage, which plays a role in refining austenite grains and improving toughness. (a) and (b) in Figure 6 are NbN, and (c) and (d) are AlN. As shown in Figure 7, 100nm-500nm NbN and AlN are precipitated during the finishing rolling process. The point indicated by the lower right arrow in (a) is NbN, and the point indicated by the upper left arrow is AlN, which plays a role in increasing the austenite defect density, refining the room temperature structure, and thus improving toughness. At the same time, the above-mentioned precipitated phases can also play a precipitation strengthening role.

(3) In order to ensure the fineness of the core structure during cooling after rolling and reduce the unevenness between the core and the surface, the following measures are taken:

① After the round steel is finished rolled, it is quickly cooled by water. After 6 sets of strong water cooling, the surface temperature of the round steel is cooled to 570℃. During this period, air cooling is interspersed. The surface temperature drop of the round steel after each set of strong water cooling is not more than 100℃. This can quickly reduce the temperature of the core of the round steel and inhibit the structural transformation of the core; at the same time, it forces the core to precipitate more fine and dispersed VC at a lower temperature, improve the strength and toughness of the core, and reduce the gap between the core and the surface. In addition, it can avoid the appearance of non-equilibrium structures such as bainite and martensite on the surface of the round steel due to too low temperature.

②After that, it was sprayed with two groups of water mist to keep the surface of the round steel at 570℃. The heat of the core was further taken away, and the surface and core of the round steel were cooled to 570℃, and the structural transformation began. The amount of ferrite in the core and the surface was close, the spacing between pearlite lamellae was close, the strength and toughness of the core and the surface were improved, and the uniformity was improved.

③ The round steel is cooled to 330℃ on the surface by a step-type cooling bed with straightening function, and then put into the pit for 48 hours of heat preservation to become the finished product. The phase change is completed, the deformation of the round steel is reduced, and the internal stress is released at the same time.

The main purpose of the post-rolling cooling stage measures is to refine the VC size and pearlite lamellar spacing, and improve the strength and cross-sectional uniformity of round steel. As shown in Figure 8, 5nm-20nm VC is precipitated in this stage, which mainly plays a role in precipitation strengthening. As shown in Figure 9, the pearlite lamellar spacing morphology of the finished round steel.

As shown in Figure 2, it is the microstructure diagram of the round steel finally obtained in this embodiment at the 1/4 radius position from the surface and the center. It can be seen from the figure that the austenite grains of the rolled material are significantly refined, and the grain sizes at the 1/4 radius position from the surface and the center are 8.0 and 7.0 respectively, and the extreme difference in grain size between the center and the surface is only 1 level.

Example 2

A method for cooperatively controlling the strength, toughness and uniformity of ultra-large cross-section non-quenched and tempered steel by multi-stage precipitation and controlled rolling and controlled cooling, wherein the continuous casting billet used has the following chemical composition by mass percentage: 0.40% C, 0.46% Si, 1.22% Mn, 0.01% P, 0.02% S, 0.022% Ti, 0.035% Nb, 0.15% Al, 0.18% V, 0.016% N, and the remainder is Fe and unavoidable impurities, wherein Ti/N=1.375, (Ti+Nb+Al)/N=12.94. The cross-sectional size of the continuous casting billet is 800mm×800mm.

A method for multi-stage precipitation and controlled rolling and controlled cooling to coordinately control the strength, toughness and uniformity of ultra-large cross-section non-quenched and tempered steel. The continuous casting billet is heated by a walking beam heating furnace, wherein the second heating stage is 1250℃, the soaking stage is 1220℃, and the second heating stage + soaking stage are kept warm for 6 hours. The walking beam heating furnace is a heating furnace for the hot rolling line, and the furnace is divided into a preheating section, a heating stage, a second heating stage and a soaking stage. After water descaling, water cooling is performed until the surface temperature reaches 1070℃, and the billet is rolled by a reversible rough rolling mill. The first two passes of rolling press down the two vertical surfaces of the continuous casting billet by 10mm respectively, and the latter two passes press down 180mm alternately, for a total of 6 passes. The four continuous sides of the continuous casting billet are named A, B, C, and D in turn. The first two passes are to roll the adjacent A and B surfaces, which are vertical. The subsequent 6 passes are also to roll the A and B surfaces alternately, or the C and D surfaces alternately, because when rolling the A surface, the A surface and the C surface are actually the same. There is one roller on the top and one on the bottom. When rolling the A side, it is also rolling the C side. That is to say, one pass rolls two sides. AC is a group that realizes rolling at the same time, and BD is a group that realizes rolling at the same time. The subsequent passes are rolled into square billets with a side length of 400mm, and the final rolling temperature is 1090℃. The intermediate billet is water-cooled until the surface temperature reaches 940℃, and then it enters the finishing continuous rolling unit to be rolled into round steel with a diameter of 280mm. The final rolling temperature is 815℃. After 8 groups of interval strong water penetration, water mist is sprayed to the surface of 560℃ and then air-cooled on the cooling bed. The water cooling time for each group is about 3-5s. In this embodiment, the water cooling time for each group is 5s. It is related to the amount of water. The water cooling time can be shortened if the amount of water is large, and the surface is cooled to 560℃ after each group of water cooling. The water temperature is room temperature; the water mist spraying is continuous, and the total time is about 2-3min. The total time of water mist spraying in this embodiment is 3min. The water mist is at room temperature. There is no time interval or requirement between two groups of water mist, as long as the total time is 2-3 minutes. Water mist spraying does not necessarily have to be 2 groups. The surface is cooled to 560℃ after water mist spraying; it is put into the insulation pit at 330℃ and kept for 48 hours to become the finished product.

The microstructure level and mechanical properties of the round steel core and 1/4 radius position in this embodiment are shown in Table 1.

As shown in Figure 3, it is the microstructure diagram of the round steel at the 1/4 radius position from the surface and the center of the round steel finally obtained in this embodiment. It can be seen from the figure that the austenite grains of the rolled material are significantly refined, and the grain sizes at the 1/4 radius position from the surface and the center are 8.0 and 6.5 respectively, and the extreme difference in grain size between the center and the surface is only 1.5.

Comparative Example 1

A non-quenched and tempered steel with an ultra-large cross-section, the continuous casting billet used has the following chemical composition by mass percentage: 0.39% C, 0.58% Si, 1.52% Mn, 0.01% P, 0.03% S, 0.040% Ti, 0.030% Nb, 0.02% Al, 0.30% V, 0.008% N, the balance is Fe and unavoidable impurities, the cross-sectional size of the continuous casting billet is 430 mm×480 mm. Among them, Ti/N=5, greater than 3.4; (Ti+Nb+Al)/N=11.25, less than 12.5.

A kind of ultra-large cross-section non-quenched and tempered steel, the continuous casting billet is heated by a walking beam heating furnace, wherein the second heating stage is 1250℃, the soaking stage is 1230℃, and the second heating stage + soaking stage are kept warm for 2.0h. After water descaling, water cooling is carried out until the surface temperature reaches 1150℃, and then rolled by a reversible rough rolling mill. The first two rolling passes press down the two vertical surfaces of the continuous casting billet by 10mm respectively, and the latter two surfaces press down 50mm alternately, for a total of 12 passes. The subsequent passes are rolled into a square billet with a side length of 240mm, and the final rolling temperature is 1080℃. The intermediate billet is rolled into a round steel with a diameter of 200mm at 1050℃ in the finishing continuous rolling unit. The final rolling temperature is 880℃. After two sets of strong water penetration, the surface is air-cooled on the cooling bed at 760℃, and then kept warm in the pit at 430℃.

The microstructure level and mechanical properties of the round steel at the center and 1/4 radius position of Comparative Example 1 are shown in Table 1.

As shown in Figure 1, it is the microstructure of the rolled material at the 1/4 radius position from the surface and the center of the comparative example 1. From the figure, it can be seen that the grain sizes at the 1/4 radius position from the surface and the center of the rolled material are 6.5 and 4.0 respectively. The core structure is coarse, the grain size difference between the center and the surface is large, and it is uneven.

The microstructure levels and mechanical properties of the round steel core and 1/4 radius position of Examples 1 to 2 and Comparative Example 1 are shown in Table 1.

Table 1 below is a performance comparison result of Examples 1-2 and Comparative Example 1.

Table 1: Tissue levels and mechanical properties at the core and 1/4 radius

Example 3

A method for cooperatively controlling the strength, toughness and uniformity of ultra-large cross-section non-quenched and tempered steel by multi-stage precipitation and controlled rolling and controlled cooling, wherein the continuous casting billet used has the following chemical composition by mass percentage: 0.35% C, 0.30% Si, 1.10% Mn, 0.02% P, 0.01% S, 0.015% Ti, 0.03% Nb, 0.10% Al, 0.10% V, 0.008% N, and the balance is Fe and unavoidable impurities, wherein Ti/N=1.875, (Ti+Nb+Al)/N=18.125. The cross-sectional size of the continuous casting billet is 400mm×400mm.

A method for multi-stage precipitation and controlled rolling and controlled cooling to coordinately control the strength, toughness and uniformity of ultra-large cross-section non-quenched and tempered steel. The continuous casting billet is heated by a walking beam heating furnace, wherein the second heating stage is 1230°C and the soaking stage is 1210°C, and the second heating stage + soaking stage are kept warm for 5 hours. After water descaling, the billet is water-cooled to a surface temperature of 1100°C, and rolled by a reversible rough rolling mill. The first two rolling passes press down the two vertical surfaces of the continuous casting billet by 5mm respectively, and the latter two surfaces are alternately pressed down by 80mm, for a total of 4 passes. The subsequent passes are rolled into a square billet with a side length of 230mm, and the final rolling temperature is 1050°C. The intermediate billet is water-cooled to a surface temperature of 900°C and then rolled into a round steel with a diameter of 160mm in the finishing continuous rolling mill. The final rolling temperature is 780°C. After 7 groups of interval strong water penetration, the water mist is sprayed to a surface temperature of 550°C and then air-cooled on the cooling bed. In this embodiment, the water cooling time for each group is 3s, and the total water mist spraying time is 2min. After being kept in the pit at 300℃ for 50 hours, it becomes the finished product.

The finished product obtained in this embodiment has a longitudinal tensile strength of 900 MPa at the 1/4 radius position, a yield strength of 650 MPa, an elongation after fracture of 15%, and an impact energy KU ₂ of 40 J; the longitudinal tensile strength of the core is 920 MPa, the yield strength is 669 MPa, the elongation after fracture is 15.5%, and the impact energy KU ₂ is 42 J.

Example 4

A method for cooperatively controlling the strength, toughness and uniformity of ultra-large cross-section non-quenched and tempered steel by multi-stage precipitation and controlled rolling and controlled cooling, wherein the continuous casting billet used has the following chemical composition by mass percentage: 0.45% C, 0.90% Si, 1.60% Mn, 0.015% P, 0.04% S, 0.030% Ti, 0.04% Nb, 0.30% Al, 0.20% V, 0.020% N, and the remainder is Fe and unavoidable impurities, wherein Ti/N=1.5, (Ti+Nb+Al)/N=18.5. The cross-sectional size of the continuous casting billet is 1000mm×1000mm.

A method for multi-stage precipitation and controlled rolling and controlled cooling to coordinately control the strength, toughness and uniformity of ultra-large cross-section non-quenched and tempered steel. The continuous casting billet is heated by a walking beam heating furnace, wherein the second heating stage is 1240℃, the soaking stage is 1220℃, and the second heating stage + soaking stage are kept warm for 7h. After water descaling, the billet is water-cooled to a surface temperature of 1090℃, and rolled by a reversible rough rolling mill. The first two rolling passes press down the two vertical surfaces of the continuous casting billet by 7mm respectively, and the latter two surfaces are alternately pressed down by 250mm, for a total of 6 passes. The subsequent passes are rolled into a square billet with a side length of 450mm, and the final rolling temperature is 1090℃. The intermediate billet is water-cooled to a surface temperature of 950℃ and then rolled into a round steel with a diameter of 300mm in the finishing continuous rolling unit. The final rolling temperature is 820℃. After 8 groups of interval strong water penetration, the water mist is sprayed to a surface temperature of 600℃, and then air-cooled on the cooling bed, and then kept in the pit at 350℃ for 48h to become a finished product.

The finished product obtained in this embodiment has a longitudinal tensile strength of 1027 MPa at the 1/4 radius position, a yield strength of 779 MPa, an elongation after fracture of 17%, and an impact energy KU ₂ of 46 J; the longitudinal tensile strength of the core is 1050 MPa, the yield strength is 800 MPa, the elongation after fracture is 19%, and the impact energy KU ₂ is 50 J.

Example 5

The only difference between this embodiment and Embodiment 4 is: a method for coordinated regulation of strength, toughness and uniformity of ultra-large cross-section non-quenched and tempered steel by multi-stage precipitation and controlled rolling and controlled cooling, wherein the continuous casting billet used has the following chemical composition in mass percentage: 0.40% C, 0.75% Si, 1.35% Mn, 0.015% P, 0.02% S, 0.028% Ti, 0.04% Nb, 0.30% Al, 0.20% V, 0.0082% N, and the remainder is Fe and unavoidable impurities, wherein Ti/N=3.37, (Ti+Nb+Al)/N=44.88.

The finished product obtained in this embodiment has a longitudinal tensile strength of 1005 MPa, a yield strength of 766 MPa, an elongation after fracture of 16.1%, and an impact energy KU ₂ of 40 J at the 1/4 radius position; the longitudinal tensile strength of the core is 1022 MPa, the yield strength is 778 MPa, the elongation after fracture is 17%, and the impact energy KU ₂ is 45 J.

It should be understood that in order to streamline the present disclosure and aid in understanding one or more of the various inventive aspects, in the above description of exemplary embodiments of the present invention, various features of the present invention are sometimes grouped together into a single embodiment, figure, or description thereof. However, this disclosed method should not be interpreted as reflecting the intention that the claimed invention requires more features than those expressly recited in each claim. Rather, as reflected in the claims, inventive aspects lie in less than all of the features of the previously disclosed embodiments. Therefore, the claims that follow the detailed description are hereby expressly incorporated into the detailed description, with each claim itself serving as a separate embodiment of the present invention.

Although the present invention has been described according to a limited number of embodiments, it will be apparent to those skilled in the art, with the benefit of the above description, that other embodiments may be envisioned within the scope of the invention thus described. In addition, it should be noted that the language used in this specification is selected primarily for readability and didactic purposes, rather than for explaining or defining the subject matter of the present invention. Therefore, many modifications and variations will be apparent to those skilled in the art without departing from the scope and spirit of the appended claims. The disclosure of the present invention is illustrative, not restrictive, with respect to the scope of the present invention, which is defined by the appended claims.

The above is only a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principle of the present invention. These improvements and modifications should also be regarded as the scope of protection of the present invention.

Claims (10)

1. A super-large cross-section non-quenched and tempered steel, characterized in that, in terms of mass percentage, the chemical composition is as follows: C: 0.35%-0.45%, Si: 0.30%-0.90%, Mn: 1.10%-1.60%, V: 0.10%-0.20%, Nb: 0.03%-0.04%, Ti: 0.015%-0.030%, Al: 0.10%-0.30%, N: 0.008%-0.020%, S: 0.01%-0.04%, P≤0.02%, and the remainder is Fe and unavoidable impurities; wherein the Ti/N mass percentage ratio is less than 3.4, and the (Ti+Nb+Al)/N mass percentage ratio is greater than 12.5.
2. The super-large cross-section non-quenched and tempered steel according to claim 1 is characterized in that the mass percentage of Ti is 0.020% to 0.028%.
3. The super-large cross-section non-quenched and tempered steel according to claim 1 is characterized in that the diameter of the super-large cross-section non-quenched and tempered steel is 160-300 mm.
4. A method for cooperatively controlling the strength, toughness and uniformity of the ultra-large cross-section non-quenched and tempered steel by multi-stage precipitation and controlled rolling and controlled cooling for the ultra-large cross-section non-quenched and tempered steel according to any one of claims 1 to 3, characterized in that it comprises the following steps:

   S1: Continuous heating before rolling of continuous casting billet, the temperature of heating stage 2 is 1230℃～1250℃, the soaking stage is 1210℃～1230℃, and the total holding time of heating stage 2 and soaking stage is not less than 5h;

   S2: After the continuous casting billet is removed from the furnace, it is descaled by water, cooled by water or heated to a surface temperature below 1100°C before rolling begins;

   S3: Use a reversible rough rolling mill to roll the billet. The first two passes are to roll the two vertical surfaces of the continuous casting billet, and each pass is pressed down by 5-10mm. The following 4 to 6 consecutive passes are alternately rolled on the two vertical surfaces of the continuous casting billet, and the deformation amount of each pass is ≥20%. The intermediate billet is rolled, and the final rough rolling temperature is greater than 1050℃;

   S4: The intermediate billet is water-cooled or heated to a surface temperature of 900-950°C;

   S5: The intermediate billet is rolled into round steel by continuous finishing rolling, and the final rolling temperature is 780℃～820℃;

   S6: After the round steel is finally rolled, it is cooled by water. After 6-8 groups of strong water cooling, the surface temperature is cooled to 550-600℃. During this period, air cooling is interspersed. The temperature return time is 5-10s each time. The temperature return will be performed between every two groups of water tanks. The surface temperature drop of each group of strong water-cooled round steel after the temperature return is no more than 100℃. After that, it is sprayed with water mist to keep the surface temperature of the round steel at 550-600℃.

   S7: After the round steel is cooled to 300-350℃ on the surface by a walking cooling bed with straightening function, it is put into the pit for insulation for at least 48 hours to become the finished product.
5. The method according to claim 4 is characterized in that the continuous casting billet is a square billet or a rectangular billet with a side length of 400-1000 mm; and the intermediate billet is a square billet with a side length of 230-450 mm.
6. The method according to claim 4 is characterized in that the longitudinal tensile strength of the finished product at 1/4 radius and core is 900-1050MPa, the yield strength is 650-800MPa, the elongation after fracture is 15-19%, and the impact energy KU ₂ is 40-50J.
7. The method according to claim 4 is characterized in that, in S1, 3-5 μm of TiN is retained during the continuous casting billet heating stage.
8. The method according to claim 4 is characterized in that, in S3, TiN and NbN of 1-3 μm are precipitated during the rough rolling process.
9. The method according to claim 4 is characterized in that, in S4, TiN, NbN and AlN of 500nm-1μm are precipitated during water cooling or temperature waiting of the intermediate billet.
10. The method according to claim 4 is characterized in that, in S5, 100nm-500nm of NbN and AlN are precipitated during the finish rolling process; and in S6, 5nm-20nm of VC is precipitated during the cooling stage after rolling.

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